With the increasing popularity of the Internet and other content-heavy electronic communication systems, there has been a substantial need for reliable and affordable high bandwidth mediums for facilitating data transmissions between service providers and their customers. In relation to the requirement that such mediums be affordable to consumers, a cost-effective manner for providing service to customers involves using infrastructure already present in most locations. Accordingly, over recent years, the two such mediums most widely meeting these requirements include cable television (CATV) and conventional copper wire telephone systems (plain old telephone system or POTS).
Relating specifically to the adaptation of POTS telephone lines to carry data at high-bandwidth or ‘broadband’ data rates, a number of Digital Subscriber Line (DSL) standards and protocols have been proposed. DSL essentially operates by formatting signals using various Time Domain Equalization techniques to send packets over copper wire at high data rates. A substandard of conventional DSL is known as Asymmetric Digital Subscriber Line (ADSL) and is considered advantageous for its ability to provide very high data rates in the downstream (i.e., from service provider to the user) direction by sacrificing speed in the upstream direction. Consequently, end user costs are minimized by providing higher speeds in the most commonly used direction. Further, ADSL provides a system that applies signals over a single twisted-wire pair that simultaneously supports conventional POTS or Integrated Services Digital Network (ISDN) service as well as high-speed duplex (simultaneous two-way) digital data services.
Two of the proposed standards for ADSL are set forth by the International Telecommunications Union, Telecommunication Standardization Section (ITU-T). A first, conventional, ADSL standard is described in ITU-T Recommendation G.992.1—“Asymmetric Digital Subscriber Line (ADSL) Transceivers”, the body of which is incorporated herein by reference. A second, more recently proposed standard is the G.992.2 or ‘G.lite’ standard, further described in ITU-T Recommendation G.992.2—“Splitterless Asymmetric Digital Subscriber Line (ADSL) Transceivers”, also bodily incorporated by reference herein. The G.lite standard is a variant of the G.992.1 standard, with modifications directed primarily to work in a splitterless environment (i.e., without a splitter at the remote user end to separate the voice traffic from the digital data traffic).
Prior to any transmission of actual data between the CO (ADSL Transceiver Unit-Central (ATU-C)) and the remote computer (ADSL Transceiver Unit-Remote (ATU-R)), the two entities must first undergo a initialization procedure designed to familiarize the two entities with each other, identify the bandwidth capabilities for the current session, and further facilitate the establishment of a valid connection. Pursuant to ADSL standards provided by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T), these initialization procedures comprise the following: 1) a handshake procedure; 2) a transceiver training session; 3) a channel analysis session; 4) an exchange session; and finally 5) an actual data transmission session referred to as ‘showtime’.
Specifics of the handshake procedure are set forth in ITU-T Recommendation G.994.1 —“Handshake Procedures for Digital Subscriber Line (DSL) Transceivers”, the body of which is incorporated by reference herein. The handshake procedure is designed to enable peer components to initiate a communications session between each other and generally includes the exchange of several specific types of messages having predetermined formats. Examples of such messages include the following: capabilities list and capabilities list request messages; mode select and mode request messages; various acknowledge and negative acknowledge messages, etc. Each of the above messages is generally formulated by a protocol processor responsible for making sure that the requirements for protocol communication are complied with.
Because the various ITU-T recommendations identified above are designed to provide guidance to ADSL developers in various geographic locations, different circumstances may exist which impact the method with which the general recommendations are implemented. Accordingly, Annexes have been created to each specification that specifically itemize the effect of particular scenarios upon the adoption of the general recommendations. Of particular interest in the present application is the effect of a large network of conventional TCM-ISDN (Time Compression Multiplex ISDN) telephone lines on ADSL development. Annex C of the G.992.1 Recommendation directly relates to such circumstances. Due to noise and other interference generated by these ISDN systems, as well as the potential adverse impact ADSL deployment may have on these existing systems, relatively severe performance limitations have been placed upon ADSL implementation in these regions.
Crosstalk generally refers to disturbance on one twisted pair due to signals on another twisted pair. Usually, the twisted pairs are in the same cable or bundle. Crosstalk is generally characterized as NEXT or FEXT. NEXT is characterized by a disturbing pair's source being local to the disturbed pair's receiver, the disturbing pair referring to the twisted pair causing the disturbance and the disturbed pair referring to the twisted pair being disturbed. In this case, the disturbing signal starts down the disturbing pair, couples into the disturbed pair and then propagates back to the disturbed pair's receiver. FEXT is characterized by the disturbing pair's source being distant from the disturbed pair's receiver. In this case, the disturbing signal propagates down the disturbing pair, crosstalks into the disturbed pair and propagates the rest of the distance along the disturbed pair into the disturbed pair's receiver.
Handshake tones FEXT and NEXT interferences into neighboring ADSL services sharing the same bundle may cause significant signal to noise ratio drop, leading to errors that may not be handled by an error correcting system. On existing ADSL Customer premise equipment (CPE) and central office (CO) modems, these handshake interferences are not mitigated.
Therefore, there is a need in the art of ADSL systems for a more efficient method and system for reducing interferences due to handshake tones.